Interface acoustic wave device made of lithium tantalate

ABSTRACT

The invention relates to interface acoustic wave components made of lithium tantalate. In the case in which the two substrates making up the filter have the same cut and the same crystal orientation, the invention gives the best cut angles in particular for optimizing the electromechanical coupling coefficient that determines the final performance characteristics of the component produced. Curves giving the variations of the coupling coefficient and the attenuation as a function of the cut angles and of the direction of propagation are provided. The values of the main acoustic characteristics of the component for these optimum cut angles are also given. The applications of this type of device are, on the one hand, uses as a passive component such a resonator or a filter or a delay line, or as an integrated device, either in a measurement chain or in an array of devices operating according to the principle of recognition of the device by a phase code.

The field of the invention is that of interface acoustic wave devicesand especially that of filters, produced at the interface between twolithium tantalate substrates.

It is known to produce surface acoustic wave devices that use thepropagation of waves on the surface of a piezoelectric substrate. In thecase of what are called Rayleigh waves, these are generated and receivedby interdigitated comb transducers composed of interlaced electrodesbetween which a potential difference is imposed. These devices have twomain drawbacks.

Firstly, in order for the surface waves to propagate correctly on thesurface of the substrate, this surface must remain free. This conditionis obtained by encapsulation technologies for obtaining a cavity.

Secondly, the pitch of the electrodes making up the interdigitated combsis often small, of the order of a few hundred nanometers. In addition,conducting particles of very small dimensions present inside the packagemay short-circuit a transducer and disturb the normal operation of thedevice. To alleviate this drawback, it is necessary either to make thepackages for the components hermetically sealed, or to deposit a thinlayer of insulating dielectric material on the transducers. Thisoperation, called passivation, makes it possible to eliminatesensitivity to conducting particles. However, it does not make itpossible to dispense with the encapsulation operation, which is anoperation expensive to carry out.

The use of interface acoustic wave devices allows the various problemsassociated with encapsulation to be solved. In this case, use is nolonger made of the propagation of the acoustic waves on the surface ofthe substrate, but at the interface between two substrates instead. Ofcourse, this device makes it possible to obtain a passivated componentno longer requiring a cavity to be produced. Moreover, the package maybe completely eliminated.

In 1924, Stoneley demonstrated the possibility of guiding an acousticwave at the interface between two materials [Proc. Roy. Soc. London A106, 416]. These waves were regarded as being polarized in the sagittalplane.

In 1971, Maerfeld and Tournois demonstrated the existence of horizontalshear waves propagating at the surface between two materials. Thepiezoelectric case was considered [C. Maerfeld and P. Tournois, Appl.Phys. Lett. 19, 117, 1971]. The first use of this type of wave foracoustic components is disclosed in patent FR 2 145 750. The inventiondescribed uses the propagation of pure shear waves at the interfacebetween two materials, at least one of which is piezoelectric. The casein which the two materials are identical is considered. However, theabove patent makes no mention of transducers placed at the interfacebetween the two materials.

In 1983, the propagation of waves at the interface between apiezoelectric material and an isotropic material, for the purpose ofproducing packageless SAW (surface acoustic wave) devices is described,which implicitly assumes that the transducers are placed at theinterface. The coupling coefficient was also studied [Shimitzu et al.,“Stoneley waves propagating along an interface between piezoelectricmaterial and isotropic material”, 1983 IEEE U.S. Proc. pp 373-375].

More recently, in 1998, a different combination of materials wasexamined for the purpose of filtering [M. Yamaguchi, T. Yamashita, K.Hashimoto and T. Omori, “Highly piezoelectric waves in Si/SiO₂/LiNbO₃and Si/SiO₂/LiNbO₃ structures” (unpublished)].

Finally, in 1999, patent FR 2 799 906 describes filters usingtransducers at the interface between two identical piezoelectricmaterials.

In general, an interface acoustic wave device consists of two substratesdenoted by S₁ and S₂, at least one of which is piezoelectric, and of aninterface region I lying between these two substrates, as indicated inFIG. 1. In the general case, the interface region I is a structure thatcomprises at least the electro-acoustic transducers E. Electricalinterconnections coupled to said devices allow signals to be emitted andtransmitted.

The interface waves can be used to produce passive components. Ingeneral, any type of device obtained using waves propagating on thesurface of a crystal can be produced using interface waves. Inparticular, it is possible to reflect the interface waves using arraysof metal electrodes with a period equal to a half-wavelength placed atthe interface. Thus, firstly a resonator is produced, by placing aninterdigitated transducer between two reflector arrays, and secondly afilter, by coupling resonators together via electrical or acousticmeans. The directivity of a transducer is improved by interspersingreflectors therein. All applications of surface wave components aretherefore accessible, especially delay lines, band filters, resonatorsand dispersive filters. Applications of these components as measurementsensors are also possible.

Phase code devices have specific transducers characterized by adistribution of the electrodes such that it is known how to associateone particular code with a given phase code component. Remotelyinterrogable devices using radio waves employ this principle. Theoperation is as follows: a phase code is used at emission of the wave,and the wave is picked up by an antenna connected to the input of thephase-code component; conventionally, the transducer converts the signalinto a mechanical wave. Said wave propagates as far as the outputtransducer where it is then reconverted into an electrical signal andre-emitted. The received signal is analyzed and the component that hasreceived and transformed the signal is thus identified.

Interface wave devices can be used as remotely interrogable devices,especially for measurement applications, such as for the measurement ofpressure, temperature or acceleration.

The choice of structures for the interface wave components may vary verygreatly. In particular, the following combinations may be mentioned:

-   -   the substrate S₁ is piezoelectric and S₂ is not. In this case,        S₂ is chosen according to its mechanical properties so as to        make it easier to produce the components. For example, if S₁ is        made of lithium niobate or lithium tantalate, S₂ will preferably        be made of fused silica or single-crystal silicon;    -   the two substrates are both piezoelectric, but of different        nature. For example, the following combinations may be        mentioned:        -   quartz/lithium niobate        -   quartz/lithium tantalate        -   lithium tantalate/lithium niobate;    -   the two substrates are of the same nature but of different        crystal cut. FIG. 2 shows, using the IEEE 1949 conventions in        the initial orthonormal coordinate system (X,Y,Z), Z being        parallel to the optical axis of the crystal, X being defined by        the piezoelectricity of the crystal and Y being perpendicular to        (X,Z), the geometrical representation of the cut plane being        defined by two successive rotation angles φ and θ. Here, φ        corresponds to a first rotation about the Z axis, the coordinate        system obtained thus being denoted by (X′,Y′,Z′) with Z′        coincident with Z, and θ corresponds to a second rotation about        the X′ axis, the coordinate system obtained thus being denoted        by (X″,Y″,Z″), with Z′ coincident with X′. In this final        coordinate system, the direction of propagation of the acoustic        waves is then defined by a third angle Ψ representing a rotation        about the Y″ axis. As an example, with these conventions, the        values of the angles for the cuts normally used in surface waves        for ST quartz are the following:    -   φ=0°; θ=42.75°; Ψ=0°; and finally    -   the two substrates may be of the same nature and same cut. For        example, it is possible to use quartz and lithium niobate or        lithium tantalate. In this case, as a general rule the assembly        operation will be carried out with the same crystal orientation        of the two substrates.

The latter case is particularly interesting in so far as the problems ofcompatibility between the substrates S₁ and S₂, in particular thethermal expansion and assembly problems, are implicitly solved. In thiscase, the orientations of the crystal faces are chosen so as to obtainpolarizations of the same direction. of propagation. It is thus possibleto obtain, depending on the cut angle in the case of lithium tantalate,k2 values varying between 0 and 7 as indicated in FIGS. 3 a and 3 b.FIG. 3 a shows the variation of k as a function of (φ,Ψ) at zero θ andFIG. 3 b shows the variation of k2 as a function of (θ,Ψ) at zero φ. Inthese figures, the discontinuities show the regions within which nopossible propagation mode exists. The choice of the cut angle istherefore fundamental. However, the variations in the acousticcharacteristics as a function of this angle cannot be determined simply,for example by considerations regarding the crystal structure. They arealso very different from those that are obtained in the case of freematerials used for the surface acoustic waves.

U.S. Pat. No. 2,799,906 (Pierre Tournois) relating to the production ofinterface acoustic wave filters gives general recommendations mentionedby way of example and allowing optimum cut angles to be chosen.Particularly mentioned in the case of the use of lithium tantalate thatthe cuts may be taken along the Y crystallographic axis (where Y isrotated through a certain angle, for example 175°).

The invention itself proposes a selection of optimized cut angle ranges.In fact it gives precisely the tolerances on the cut angles, the set ofoptimum cut angles making it possible to obtain, in the case of lithiumtantalate, the best possible characteristics, in particular the highestvalues of the parameter k2, and finally the values of the maincharacteristics for achieving the expected performance characteristics.

The various steps in producing the components are also described.

Within this context, the subject of the invention is an interfaceacoustic wave device comprising:

-   -   a first crystal substrate made of lithium tantalate (LiTaO₃);    -   a second crystal substrate, also made of lithium tantalate;    -   these being joined together via a plane interface region serving        for the propagation of acoustic waves and comprising at least        the electroacoustic transducers;    -   interconnection means for electrically connecting said        transducers;        the substrates, referenced in an initial orthonormal coordinate        system (X,Y,Z), Z being parallel to the optical axis, X being        defined by the piezoelectricity of the crystal and Y being        perpendicular to (X,Z), having their identical cut plane and a        common crystal orientation, said cut plane being identified by        two successive angles of rotation φ and θ, φ corresponding to a        first rotation about the Z axis, the coordinate system obtained        thus being denoted by (X′,Y′,Z′) with Z′ coincident with Z, and        θ representing a second rotation about the X′ axis, the        coordinate system obtained thus being denoted by (X″,Y″,Z″),        with X″ coincident with X′, the direction of propagation of the        acoustic waves being defined by a third angle Ψ taken in said        coordinate system (X″,Y″,Z″) representing a rotation about the        Y″ axis, characterized in that, for any direction of propagation        Ψ:    -   the angles (φ,θ) lie within one of the following two angular        ranges called the (0,0,0)_(a) cut and the (60,0,0)_(a) cut:    -   (0,0,0)_(a) cut:        -   −5°≦φ≦+5°        -   −20°≦θ≦+30°    -   (60,0,0)_(a) cut:        -   +55°≦φ≦+65°        -   −30°≦θ≦+20°.

Advantageously, so as to obtain the maximum values of the couplingcoefficient and to reduce the insertion losses, it is preferable to workwithin restricted angular ranges. In this case, the angles (φ,θ,Ψ),taken within the (X″,Y″,Z″) coordinate system lie within one of thefollowing two angular ranges called the (0,0,0)_(b) cut and the(60,0,0)_(b) cut:

-   -   (0,0, 0)_(b) cut:        -   −5°≦φ≦+5°        -   −10°≦θ≦+10°        -   −5°≦Ψ≦+5°    -   (60,0,0)_(b) cut:        -   +55°≦φ≦+65°        -   −10°≦θ≦+10°        -   −5≦Ψ≦+5°.

Advantageously, the thicknesses of the two substrates are large comparedwith the operating acoustic wavelength (λ). Under these conditions, theacoustic waves remain confined within the two substrates, therebypreventing any possible perturbation from the outside. In contrast, thethickness of the interface region is chosen to be small compared to thesame operating acoustic wavelength so that the perturbations introducedby the intrinsic mechanical properties of said region are negligible.

Advantageously, the interface region is in the form of a laminatedstructure comprising at least the electroacoustic transducers and one ormore layers of dielectric material. The main advantages of these layersare that they promote either the propagation of the waves or theadhesion of the second substrate to the interface region.

Advantageously, the interface region comprises only the electroacoustictransducers, which are then etched on one of the two surfaces of thesubstrates in contact with each other, said interface region then beingreduced to an interface plane. Under these conditions, the combinationof the two substrates and of the interface region once assembled isequivalent to a single substrate within which the transducers are found.

Advantageously, the device may be used for all applications accessibleto surface acoustic wave devices, especially as a passive component suchas a resonator or a filter or a delay line or a phase code device.

Finally, it is possible to use it either in a measurement chain or in anarray of devices operating according to the principle of coded devices,such as a remotely interrogable device.

The invention will be more clearly understood and other advantages willbecome apparent on reading the description that follows, which is givenby way of nonlimiting example, together with the appended figures inwhich:

FIG. 1 gives the general configuration of an interface wave device shownin a schematic manner;

FIG. 2 gives the geometrical representation of the cut angle defined bythe angles φ and θ and the representation of the direction ofpropagation in the coordinate system of the cut angle;

FIGS. 3 a and 3 b give the variations of k2 as a function of the cutangle and of the direction of propagation for lithium tantalate in thefollowing two configurations: zero θ and zero φ;

FIGS. 4 a and 4 b give the map of the variations of the couplingcoefficient k2 and of the insertion losses as a function of thevariations in θ and φ for the (0,0,0)_(a) cut at zero Ψ;

FIGS. 5 a and 5 b give the map of the variations of the couplingcoefficient k2 and of the insertion losses as a function of thevariations of θ and φ for the (60,0,0) a cut at zero Ψ;

FIGS. 6 a, 6 b and 6 c give the variations of the coupling coefficientk2 and of the insertion losses near the (0,0,0)_(a) cut as a function ofthe variations of one of the three angles θ, φ and Ψ, the other twoangles being chosen to be constant; and

FIGS. 7 a, 7 b and 7 c give the variations of the coupling coefficientk2 and of the insertion losses near the (60,0,0)_(a) cut as a functionof the variations of one of the three angles θ, φ and Ψ, the other twoangles being chosen to be constant.

To obtain good coupling coefficients k2, typically greater than 3%, andlow attenuations for all directions of propagation of the waves in theplane of the interface, the cut angles (φ,θ) fall within the followingangular ranges called the (0,0,0)_(a) cut and the (60,0,0)_(a) cut(FIGS. 4 a, 4 b, 5 a and 5 c):

-   -   (0,0,0)_(a) cut:        -   −5≦φ≦+50°        -   −20°≦θ≦+30°;    -   (60,0,0)_(a) cut:        -   +55°≦φ≦+65°        -   −30°≦θ≦+20°.

To obtain optimized coupling coefficient k2 typically greater than 6%and low attenuations, typically less than 2×10⁻³, the cut angles (φ,θ)and the direction of propagation Ψ fall within the following rangescalled the (0,0,0)_(b) cut and the (60,0,0)_(b) cut (FIGS. 4 a, 4 b, 5 aand 5 c):

-   -   (0,0,0)_(b) cut:        -   −5°≦φ≦+5°        -   −10°≦θ≦+10°        -   −5°≦Ψ≦+5°    -   (60,0,0)_(b) cut:        -   +55°≦φ≦+65°        -   −10°≦θ≦+10°        -   −5°≦Ψ≦+5°

The table below gives, for each range, the cut angle in the coordinates(φ,θ,Ψ) allowing the maximum value of the coupling coefficient k2 to beobtained. The values of the four main parameters are given for thisangle. Orientation Maximum Velocity Attenuation CFT in (φ, θ, ψ) k2 in %in ms⁻¹ in dBλ⁻¹ ppmC⁻¹ Range 1 (0, 0, 0) 6.8 4073 −3 × 10⁻⁶ −22 Range 2(60, 0, 0) 6.7 4073 −3 × 10⁻⁶ −41

The variations of k2 about these maximum values are described by FIGS. 6a to 7 c.

The production of an interface wave component made of lithium tantalatecomprises the following steps:

-   -   production of the interface region on one of the two substrates,        this production necessarily including the substep of producing        the electrode combs;    -   assembly and cutting of the two substrates; and    -   production of the electrical interconnections.

The electrode combs are produced either by what is called aburied-electrode process or what is called a deposited-electrodeprocess.

In the first case, the main steps of the process are:

-   -   production of a plane cut of the first lithium tantalate        substrate at the cut angle adopted;    -   etching of the locations of the electrodes;    -   deposition of the material intended to produce the electrodes,        which may be made of pure aluminum or an aluminum alloy for        limiting thermal migration, such as copper-titanium; and finally    -   planarization so as to leave the material only at the locations        of the etching.

In the second case, the main steps of the process are:

-   -   deposition on the plane substrate of the material intended to        produce the electrodes;    -   then cutting of this material in order to leave just the        electrodes;    -   deposition of an insert layer C between the electrodes; and    -   surfacing of the layer C so as to obtain a plane layer with the        thickness of the electrodes.

In an alternative embodiment, the process may start by depositing thelayer C, producing, in this layer, openings at the locations of theelectrodes, depositing the material of the electrodes on this layer and,finally, leveling-off so as to obtain the previous result. If thethickness of the layer is small enough compared with the acousticwavelength, the electroacoustic characteristics of the assembly are onlybarely modified.

Additional layers of dielectric material may then be added fortechnological or acoustic reasons in order to complete the interfaceregion. These layers may especially promote the bonding between the twosubstrates. However, the final thickness of I must remain small comparedwith the operating acoustic wavelength if it is desired to preserve theproperties due to the cut angles of the substrates.

The second substrate, which was also cut in lithium tantalate at thesame cut angle as the first substrate, is then attached to saidsubstrate at the end of the electrode fabrication process. This assemblymay be carried out either by molecular bonding or by anodic bonding.Geometrically, the second substrate has the same crystal orientation asthe first.

Next, the electrical interconnections are produced. There are severalpossible implantations. Mention may be made, by way of nonlimitingexamples, of:

-   -   production of the interconnection in the plane of the interface        region;    -   production of the interconnection through one of the two        substrates.

1. An interface acoustic wave devices comprising: a first crystalsubstrate (S₁) made of lithium tantalate (LiTaO₃); a second crystalsubstrate (S₂), also made of lithium tantalate; said first and secondcrystal substrates being joined together via a plane interface region(I) serving for the propagation of acoustic waves and comprising atleast the electroacoustic transducer; interconnection means forelectrically connecting said transducers; the substrates, referenced inan initial orthonormal coordinate system (X,Y,Z) Z being parallel to theoptical axis, X being defined by the piezoelectricity of the crystal andY being perpendicular to (X,Z), having their identical cut plane and acommon crystal orientation, said cut plane being identified by twosuccessive angles of rotation φ and θ, φ corresponding to a firstrotation about the Z axis, the coordinate system obtained thus beingdenoted by (X′,Y′,Z′) with Z′ coincident with Z, and θ representing asecond rotation about the X′ axis, the coordinate system obtained thusbeing denoted by (X″,Y″,Z″), with X″ coincident with X′, the directionof propagation of the acoustic waves being defined by a third angle Ψtaken in said coordinate system (X″,Y″,Z″) representing a rotation aboutthe Y″ axis, characterized in that, for any direction of propagation Ψ:the cut angles (φ,θ) lie within one of the following two angular rangescalled the (0,0,0)_(a) cut and the (60,0,0)_(a):−(0, 0, 0)_(a)  cut: − 5^(∘) ≤ φ ≤ +5^(∘) − 20^(∘) ≤ θ ≤ +30^(∘) − (60, 0, 0)_(a)  cut: + 55^(∘) ≤ φ ≤ +65^(∘) − 30^(∘) ≤ θ ≤ +20^(∘).2. The acoustic wave device as claimed in claim 1, in that the angles(φ,θ,Ψ), taken within the (X″,Y″,Z″) coordinate system lie within one ofthe following two angular ranges called the (0,0,0)_(b) cut and the(60,0,0)_(b) cut:−(0, 0, 0)_(b)  cut: − 5^(∘) ≤ φ ≤ +5^(∘) − 10^(∘) ≤ θ ≤ +10^(∘) − 5^(∘) ≤ ψ ≤ +5^(∘) − (60, 0, 0)_(b)  cut: + 55^(∘) ≤ φ ≤ +65^(∘) − 10^(∘) ≤ θ ≤ +10^(∘) − 5^(∘) ≤ ψ ≤ +5^(∘).3. The device as claimed in claim 1, in that the thicknesses of the twosubstrates are large compared with the operating acoustic wavelength (λ)and the thickness of the interface region is small compared with theoperating acoustic wavelength (λ).
 4. The device as claimed in claim 1,in that the interface region is in the form of a laminated structurecomprising at least the electroacoustic transducers and one or morelayers of dielectric material.
 5. The device as claimed in claim 1 inthat the interface region comprises only the electroacoustictransducers, which are then etched on one of the two surfaces of thesubstrates in contact with each other, said interface region then beingreduced to an interface plane.
 6. The device as claimed in claim 1 inthat said device is a passive component such as a resonator or a filteror a delay line or a phase code device.
 7. A measurement sensor composedof a chain of devices, including in that said chain comprises at leastone device as claimed in claim
 1. 8. A device operating according to thephase code principle and composed of a chain of devices, characterizedin that said chain comprises at least one device as claimed in claim 1.